The instant invention is directed to a composition capable of forming a superior battery separator membrane and to a process of forming the membrane from the subject composition.
Alkaline battery systems, because of their high energy density, have great potential for replacing the more conventional lead-acid battery system in a number of terrestrial applications and/or where a light, portable energy source is required. Typical electrode combinations of such battery systems include silver-zinc, nickel-cadmium, and nickel-zinc. The potential of alkaline batteries utilizing nickel-zinc electrode combinations has not been fully realized due to the limitation of the repeated cycling without an irreversible loss of capacity upon repeated recharge. This limitation is due to the zinc electrode and the failure of the battery separator to inhibit zinc dendrite formation between the zinc and nickel electrodes which leads to battery failure.
One of the recognized key components in extending the life and efficiency of the battery is its separator. The separator is a membrane located between the plates which freely permits electrolytic conduction. Contact between plates may be due to imperfections in the plate structure or due to warping or wrinkling of the plate during use. Such macro deformations are readily inhibited by any type of sheet meterial which is coextensive with that of the plates. Contact may also occur due to the formation of dendrites or localized needle like growths on an electrode, such as zinc dendrites formed on a zinc electrode in an alkaline nickel-zinc battery system. These dendrites bridge the gap between electrodes of opposite polarity either by puncturing the separator membrane located in the gap, or by passing through the pores of the separator. The high degree of solubility of zinc oxide in alkaline electrolytes normally permits extensive loss of active material from the negative electrode through deposition of the zinc oxide in the separator pores and onto the positive electrode. These factors cause shorting out of the battery system and significantly reduce its effective life. The ability to produce a separator membrane which can effectively act as a dendristatic diaphragm is a required criteria for forming an effective battery system.
A battery separator which is capable of increasing the efficiency of a battery system and cause it to have a high energy density is highly desired, especially with respect to alkaline battery systems. It is generally agreed that such separators should be (a) resistant to degradation by the alkaline electrolyte and by oxidation due to nascent oxygen, (b) be very thin, (c) exhibit a high degree of inhibition to dendrite formation and growth, and (d) exhibit a high degree of electrolytic conductivity.
A considerable amount of effort has been directed to providing satisfactory separator materials for secondary alkaline battery systems, which illustrates the difficulty which has been encountered in providing the many diverse characteristics required for efficient functioning as a separator. Microporous separators, that is those that have discrete pores of from about 100 to 5000 Angstroms, usually in the form of a tortuous network, exhibit a high degree of electrolyte permeability and, therefore, a high degree of electrical conductivity. However, due to their porosity, such separators lack the ability to inhibit dendritic shorting. The zinc either deposits in the pores of the separator to eventually cause shorting between the positive and negative electrode pair or replates onto the zinc electrode in the form of trees or needles (dendrites) which form a bridge between electrodes of opposite polarity.
Separators which have been developed range from various organic microporous films or semi-permeable membranes to relatively rigid layers of inorganic, often ceramic, particles bonded together in some fashion. A further type of separator which has developed involves inorganic particles contained in an organic matrix. Despite the considerable effort in this field, the development of a viable separator material for secondary alkaline battery systems remains a primary obstacle to widespread utilization of such systems.
Microporous separators, such as those described in U.S. Pat. No. 4,287,276, are formed from an organic polymer matrix and contain inorganic particles utilize a wicking phenomenon due to the presence of inorganic material to carry the electrolyte through the separator. Some of the inorganic material or a secondary organic material in the matrix may be removed to provide a porous matrix to reduce the resistivity of such separators.
Separators have also been proposed which are in the form of a membrane, that is of a sheet product having virtually no or very low porosity. The pore size of such membrane separators is normally less than about 50 Angstroms and, therefore, readily inhibit dendrite penetration. However, materials, such as polyethylene, used to form such separators exhibit high resistivity (low conductivity), poor wetting and poor stability.
Modification of polyethylene membranes has been attempted to overcome the above discussed defects. Copolymers formed from polyethylene grafted by irradiation with a polar graft-polymerizable monomers, such as acrylic acid or methacrylic acid, have been suggested in U.S. Pat. Nos. 3,427,206; 3,615,865; 3,892,594; 3,928,497; 4,122,133; 4,230,549 and elsewhere as suitable materials to produce membrane separators. As the acrylic acid is grafted only in the amorphous regions of the polyethylene and not in the crystalline regions, the resulting separators exhibit irregular properties and instability as shown by high weight loss in accelerated oxidation tests.
British patent application GB No. 2,005,290A describes battery separators formed from material prepared by copolymerizing ethylene with small amounts of acrylic acid. Such materials have the defects of not being readily processable into sheet products, especially by commercially desired continuous methods, and of exhibiting high resistivity.
It is highly desired to form a membrane separator capable of use in an alkaline battery system. It is further desired to form a separator which is stable to oxidation and other conditions normally encountered in alkaline battery systems. It is still further desired to provide a separator which is capable of inhibiting dendristatic growth yet which has high conductivity. It is still further desired to produce a composition capable of readily forming into thin sheet form.